Engines may use boosting devices, such as turbochargers, to increase engine power density. However, engine knock may occur due to increased combustion temperatures. Knock is especially problematic under boosted conditions due to high charge temperatures. The inventors herein have recognized that utilizing an engine system with a split exhaust system, where a first exhaust manifold routes exhaust gas recirculation (EGR) to an intake of the engine, upstream of a compressor of the turbocharger, and where a second exhaust manifold routes exhaust to a turbine of the turbocharger in an exhaust of the engine, may decrease knock and increase engine efficiency. In such an engine system, each cylinder may include two intake valves and two exhaust valves, where a first set of cylinder exhaust valves (e.g., scavenge exhaust valves) exclusively coupled to the first exhaust manifold may be operated at a different timing than a second set of cylinder exhaust valves (e.g., blowdown exhaust valves) exclusively coupled to the second exhaust manifold, thereby isolating a scavenging portion and blowdown portion of exhaust gases. The timing of the first set of cylinder exhaust valves may also be coordinated with a timing of cylinder intake valves to create a positive valve overlap period where fresh intake air (or a mixture of fresh intake air and EGR), referred to as blowthrough, may flow through the cylinders and back to the intake, upstream of the compressor, via an EGR passage coupled to the first exhaust manifold. Blowthrough air may remove residual exhaust gases from within the cylinders (referred to as scavenging). The inventors herein have recognized that by flowing a first portion of the exhaust gas (e.g., higher pressure exhaust) through the turbine and a higher pressure exhaust passage and flowing a second portion of the exhaust gas (e.g., lower pressure exhaust) and blowthrough air to the compressor inlet, combustion temperatures can be reduced while improving the turbine's work efficiency and engine torque.
However, the inventors herein have recognized potential issues with such systems. As one example, at a part throttle condition (where an intake throttle is at least partially closed), flow in the EGR passage may be reversed and intake air may be introduced into engine cylinders via the EGR passage. This may cause decreased mixing and decreased cylinder balance. The inventors have further realized that an EGR valve disposed in the EGR valve may be closed to reduce the reverse flow through the system. However, this may increase pressures within the scavenge exhaust manifold and increase residual gases remaining in the engine cylinders.
In one example, the issues described above may be addressed by a method, comprising: routing intake air from an intake passage to a first exhaust manifold coupled to a first set of cylinder exhaust valves via an exhaust gas recirculation (EGR) passage; heating the intake air as it passes through an EGR cooler in the EGR passage; routing the heated intake air to an intake manifold, downstream of an intake throttle, via a flow passage coupled between the first exhaust manifold and the intake manifold; and exhausting combustion gases via a second set of cylinder exhaust valves to a second exhaust manifold coupled to an exhaust passage. As one example, this routing of the intake air may occur responsive to an amount of opening of the intake throttle being less than a threshold amount of opening (e.g., at a part throttle condition). In this way, pumping work of the cylinders may be reduced during the part throttle condition. Further, heating the intake air via the EGR cooler may increase MAP and reduce intake pumping, as well as increase fuel economy and reduce emissions. This operation may also increase the mixing of EGR from each cylinder with incoming intake air, thereby reducing an impact of any one cylinder on EGR mixing and reducing pushback and manifold tuning.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.